The present invention relates to curable resin compositions. In particular, the invention relates to curable resin compositions exhibiting reduced volume change on curing.
Thermosetting resins in general, and unsaturated polyester resins in particular, are useful in a variety of applications, including structural automotive parts, building and construction components, and boat hulls. In a typical sheet molding compound (SMC) formulation, the unsaturated polyester comprises only about 8 to 15% of the total formulation. Other components include a crosslinking agent such as styrene monomer, fillers such as calcium carbonate, glass fiber, thickeners, mold release agents, low profile additives, initiators, co-promoters, and inhibitors. Unsaturated polyester resins may exhibit excellent physical properties and solvent resistance, as well as good weatherability.
Notwithstanding these advantages, the commercial adoption of unsaturated polyester resins has been limited by a number of deficiencies, including (1) poor surface appearance, including fiber patterns, (2) warpage of molded parts, (3) difficulty molding to close tolerances, (4) internal cracks and voids, particularly in thick sections, and (5) notable depressions (sink marks) opposite reinforcing ribs and bosses. These deficiencies are thought to be caused by the high polymerization shrinkage from the copolymerization of the unsaturated polyester resin with the crosslinking agent. The shrinkage causes the compound to pull away from the mold surface.
In a normal high-temperature-curing cycle, the liquid resin is heated to temperatures in excess of 140xc2x0 C., resulting in a thermally induced expansion. As the unsaturated polyester resin begins to crosslink and the unsaturated components become consumed there is a negative volume change due to the density difference. Once reaction is complete, the system is cooled to ambient condition, causing a further negative volume change. A neat polyester resin will typically exhibit as much as a negative 7% volume change on high-temperature curing.
Although many approaches have been taken to reduce curing shrinkage, including changes in resin and co-monomer structures, use of large amounts of filler, and even partial polymerization before molding, these approaches have been inadequate. Another approach has been the addition of certain thermoplastics to the formulation. These thermoplastics, when functioning in such a role, are commonly referred to as low-profile additives (LPAs). Known LPAs include polymethyl methacrylates, vinyl chloride-vinyl acetate copolymers, polyurethanes, and styrene-butadiene copolymers.
The generally accepted mechanism for shrinkage control relies on induced strain relief through stress cracking of the separate LPA phase within the thermoset matrix. Many known LPAs have a high molecular weight and are polar in nature to improve compatibility with the uncured resin. During curing (crosslinking), as the resin polarity decreases, the LPA is rejected from the matrix and isolated as solid domains, typically less than about 5 micrometers in size. These distinct LPA domains dispersed in the cured thermoset matrix act as strain relief sites. Thus when the strain increases it can induce preferential stress cracking through the weak thermoplastic phase, thus relieving the strain, forming voids and compensating for the overall shrinkage.
However, in low temperature curing applications, known LPAs are not as effective. Thus, there is a need for a shrinkage control and stress release agent for low-temperature curing applications.